1. Field of the Invention
The present invention relates generally to nonvolatile memory devices using a resistance material, and more particularly, but not by way of limitation, to a phase-change random access memory (PRAM) that is configured to apply a step-down voltage to wordlines during a standby mode.
2. Description of the Related Art
Nonvolatile memories using resistance materials include phase-change random access memories (PRAMs), resistive RAMs (RRAMs), and magnetic RAMs (MRAMs). While dynamic RAMs (DRAMs) or flash memories store data using charges, nonvolatile memories using resistance materials store data using a state change of a phase-change material such as chalcogenide alloy (in the case of PRAMs), a resistance change of a variable resistance material (in the case of RRAMs), or a resistance change of a magnetic tunnel junction (MTJ) thin film according to a magnetization state of a ferromagnetic material (in the case of MRAMs).
In particular, a phase-change material of a PRAM becomes a crystalline state or an amorphous state as it is cooled after being heated. The phase-change material has low resistance in the crystalline state and has high resistance in the amorphous state. Therefore, the crystalline state may be defined as set data or data 0, and the amorphous state may be defined as reset data or data 1.
A phase-change memory cell includes a variable resistance device made of a phase-change material and an access device controlling current that flows through the variable resistance device. The access device may be a transistor or a diode. A phase-change memory cell using a transistor as the access device is referred to as a transistor-controlled PRAM cell, and a phase-change memory cell using a diode as the access device is referred to as a diode-controlled PRAM cell.
Known PRAMs have many disadvantages, however. For instance, a standby current in PRAMs are larger than desired. Improved PRAM devices and other nonvolatile memories that use resistive materials are therefore needed.